Internet Engineering Task Force (IETF) B. Carpenter
Request for Comments: 5887 Univ. of Auckland
Category: Informational R. Atkinson
ISSN: 2070-1721 Extreme Networks
H. Flinck
Nokia Siemens Networks
May 2010
Renumbering Still Needs Work
Abstract
This document reviews the existing mechanisms for site renumbering
for both IPv4 and IPv6, and it identifies operational issues with
those mechanisms. It also summarises current technical proposals for
additional mechanisms. Finally, there is a gap analysis identifying
possible areas for future work.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5887.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 5887 Renumbering Still Needs Work May 20101. Introduction
In early 1996, the IAB published a short RFC entitled "Renumbering
Needs Work" [RFC1900], which the reader is urged to review before
continuing. Almost ten years later, the IETF published "Procedures
for Renumbering an IPv6 Network without a Flag Day" [RFC4192]. A few
other RFCs have touched on router or host renumbering: [RFC1916],
[RFC2071], [RFC2072], [RFC2874], [RFC2894], and [RFC4076].
In fact, since 1996, a number of individual mechanisms have become
available to simplify some aspects of renumbering. The Dynamic Host
Configuration Protocol (DHCP) is available for IPv4 [RFC2131] and
IPv6 [RFC3315]. IPv6 includes Stateless Address Autoconfiguration
(SLAAC) [RFC4862], and this includes Router Advertisements (RAs) that
include options listing the set of active prefixes on a link. The
Point-to-Point Protocol (PPP) [RFC1661] also allows for automated
address assignment for both versions of IP.
Despite these efforts, renumbering, especially for medium to large
sites and networks, is widely viewed as an expensive, painful, and
error-prone process, and is therefore avoided by network managers as
much as possible. Some would argue that the very design of IP
addressing and routing makes automatic renumbering intrinsically
impossible. In fact, managers have an economic incentive to avoid
having to renumber, and many have resorted to private addressing and
Network Address Translation (NAT) as a result. This has the highly
unfortunate consequence that any mechanisms for managing the scaling
problems of wide-area (BGP4) routing that require occasional or
frequent site renumbering have been consistently dismissed as
unacceptable. But none of this means that we can duck the problem,
because as explained below, renumbering is sometimes unavoidable.
This document aims to explore the issues behind this problem
statement, especially with a view to identifying the gaps and known
operational issues.
It is worth noting that for a very large class of users, renumbering
is not in fact a problem of any significance. A domestic or small
office user whose device operates purely as a client or peer-to-peer
node is in practice renumbered at every restart (even if the address
assigned is often the same). A user who roams widely with a laptop
or pocket device is also renumbered frequently. Such users are not
concerned with the survival of very long-term application sessions
and are in practice indifferent to renumbering. Thus, this document
is mainly concerned with issues affecting medium to large sites.
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There are numerous reasons why such sites might need to renumber in a
planned fashion, including:
o Change of service provider, or addition of a new service provider,
when provider-independent addressing is not an option.
o A service provider itself has to renumber.
o Change of site topology (i.e., subnet reorganisation).
o Merger of two site networks into one, or split of one network into
two or more parts.
o During IPv6 deployment, change of IPv6 access method (e.g., from
tunneled to native).
The most demanding case would be unplanned automatic renumbering,
presumably initiated by a site border router, for reasons connected
with wide-area routing. There is already a degree of automatic
renumbering for some hosts, e.g., IPv6 "privacy" addresses [RFC4941].
It is certainly to be expected that as the pressure on IPv4 address
space intensifies in the next few years, there will be many attempts
to consolidate usage of addresses so as to avoid wastage, as part of
the "end game" for IPv4, which necessarily requires renumbering of
the sites involved. However, strategically, it is more important to
implement and deploy techniques for IPv6 renumbering, so that as IPv6
becomes universally deployed, renumbering becomes viewed as a
relatively routine event. In particular, some mechanisms being
considered to allow indefinite scaling of the wide-area routing
system might assume site renumbering to be a straightforward matter.
There is work in progress that, if successful, would eliminate some
of the motivations for renumbering. In particular, some types of
solutions to the problem of scalable routing for multihomed sites
would likely eliminate both multihoming and switching to another ISP
as reasons for site renumbering.
Several proposed identifier/locator split schemes provide good
examples, including at least Identifier Locator Network Protocol
[ILNP], Locator/ID Separation Protocol [LISP], and Six/One [SIX-ONE]
(in alphabetical order). The recent discussion about IPv6 Network
Address Translation (IPv6 NAT) provides a separate example [NAT66].
While remaining highly contentious, this approach, coupled with
unique local addresses or a provider-independent address prefix,
would appear to eliminate some reasons for renumbering in IPv6.
However, even if successful, such solutions will not eliminate all of
the reasons for renumbering. This document does not take any
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position about development or deployment of protocols or technologies
that would make long-term renumbering unnecessary, but rather deals
with practical cases where partial or complete renumbering is
necessary in today's Internet.
IP addresses do not have a built-in lifetime. Even when an address
is leased for a finite time by DHCP or SLAAC, or when it is derived
from a DNS record with a finite time to live (TTL) value, this
information is unavailable to applications once the address has been
passed to an upper layer by the socket interface. Thus, a
renumbering event is almost certain to be an unpredictable surprise
from the point of view of any application software using the address.
Many of the issues listed below derive from this fact.
2. Existing Host-Related Mechanisms2.1. DHCP
At a high level, DHCP [RFC2131] [RFC3315] offers similar support for
renumbering for both versions of IP. A host requests an address when
it starts up, the request might be delivered to a local DHCP server
or via a relay to a central server, and if all local policy
requirements are met, the server will provide an address with an
associated lifetime, and various other network-layer parameters (in
particular, the subnet mask and the default router address).
From an operational viewpoint, the interesting aspect is the local
policy. Some sites require pre-registration of MAC (Media Access
Control) addresses as a security measure, while other sites permit
any MAC address to obtain an IP address. Similarly, some sites use
DHCP to provide the same IP address to a given MAC address each time
(this is sometimes called "Static DHCP"), while other sites do not
(this is sometimes called "Dynamic DHCP"), and yet other sites use a
combination of these two modes where some devices (e.g., servers,
Voice over IP (VoIP) handsets) have a relatively static IP address
that is provisioned via DHCP while other devices (e.g., portable
computers) have a different IP address each time they connect to the
network. As an example, many universities in the United States and
United Kingdom require MAC address registration of faculty, staff,
and student devices (including handheld computers with wireless
connections).
These policy choices interact strongly with whether the site has what
might be called "strong" or "weak" asset management. At the strong
extreme, a site has a complete database of all equipment allowed to
be connected, certainly containing the MAC address(es) for each host,
as well as other administrative information of various kinds. Such a
database can be used to generate configuration files for DHCP, DNS,
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and any access control mechanisms that might be in use. For example,
only certain MAC addresses might be allowed to get an IP address on
certain subnets. At the weak extreme, a site has no asset
management, any MAC address may get a first-come first-served IP
address on any subnet, and there is no network-layer access control.
The IEEE 802.1X standard [IEEE.802-1X] [IEEE.802-1X-REV] specifies a
connection mechanism for wired/wireless Ethernet that is often
combined with DHCP and other mechanisms to form, in effect, a network
login. Using such a network login, the user of a device newly
connecting to the network must provide both identity and
authentication before being granted access to the network. As part
of this process, the network control point will often configure the
point of network connection for that specific user with a range of
parameters -- such as Virtual LAN (VLAN), Access Control Lists
(ACLs), and Quality-of-Service (QoS) profiles. Other forms of
network login also exist, often using an initial web page for user
identification and authentication. The latter approach is commonly
used in hotels or cafes.
In principle, a site that uses DHCP can renumber its hosts by
reconfiguring DHCP for the new address range. The issues with this
are discussed below.
2.2. IPv6 Stateless Address Autoconfiguration
SLAAC, although updated recently [RFC4862], was designed prior to
DHCPv6 and was intended for networks where unattended automatic
configuration was preferred. Ignoring the case of an isolated
network with no router, which will use link-local addresses
indefinitely, SLAAC follows a bootstrap process. Each host first
gives itself a link-local address, and then needs to receive a link-
local multicast Router Advertisement (RA) [RFC4861] that tells it the
routeable subnet prefix and the address(es) of the default router(s).
A node may either wait for the next regular RA or solicit one by
sending a link-local multicast Router Solicitation. Knowing the link
prefix from the RA, the node may now configure its own address.
There are various methods for this, of which the basic one is to
construct a unique 64-bit identifier from the interface's MAC
address.
We will not describe here the IPv6 processes for Duplicate Address
Detection (DAD), Neighbour Discovery (ND), and Neighbour
Unreachability Discovery (NUD). Suffice it to say that they work,
once the initial address assignment based on the RA has taken place.
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The contents of the RA message are clearly critical to this process
and its use during renumbering. An RA can indicate more than one
prefix, and more than one router can send RAs on the same link. For
each prefix, the RA indicates two lifetimes: "preferred" and "valid".
Addresses derived from this prefix must inherit its lifetimes. When
the valid lifetime expires, the prefix is dead and the derived
address must not be used any more. When the preferred lifetime is
expired (or set to zero) the prefix is deprecated, and must not be
used for any new sessions. Thus, setting a preferred lifetime that
is zero or finite is SLAAC's warning that renumbering will occur.
SLAAC assumes that the new prefix will be advertised in parallel with
the deprecated one, so that new sessions will use addresses
configured under the new prefix.
2.3. IPv6 ND Router/Prefix Advertisements
With IPv6, a Router Advertisement not only advertises the
availability of an upstream router, but also advertises routing
prefix(es) valid on that link (subnetwork). Also, the IPv6 RA
message contains a flag indicating whether or not the host should use
DHCPv6 to configure. If that flag indicates that the host should use
DHCPv6, then the host is not supposed to autoconfigure itself as
outlined in Section 2.2. However, there are some issues in this
area, described in Section 5.1.1.
In an environment where a site has more than one upstream link to the
outside world, the site might have more than one valid routing
prefix. In such cases, typically all valid routing prefixes within a
site will have the same prefix length. Also, in such cases, it might
be desirable for hosts that obtain their addresses using DHCPv6 to
learn about the availability of upstream links dynamically, by
deducing from periodic IPv6 RA messages which routing prefixes are
currently valid. This application seems possible within the IPv6
Neighbour Discovery architecture, but does not appear to be clearly
specified anywhere. So, at present, this approach for hosts to learn
about availability of new upstream links or loss of prior upstream
links is unlikely to work with currently shipping hosts or routers.
2.4. PPP
"The Point-to-Point Protocol (PPP)" [RFC1661] includes support for a
Network Control Protocol (NCP) for both IPv4 and IPv6.
For IPv4, the NCP is known as IPCP [RFC1332] and allows explicit
negotiation of an IP address for each end. PPP endpoints acquire
(during IPCP negotiation) both their own address and the address of
their peer, which may be assumed to be the default router if no
routing protocol is operating. Renumbering events arise when IPCP
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negotiation is restarted on an existing link, when the PPP connection
is terminated and restarted, or when the point-to-point medium is
reconnected. Peers may propose either the local or remote address or
require the other peer to do so. Negotiation is complete when both
peers are in agreement. In practice, if no routing protocol is used,
as in a subscriber/provider environment, then the provider proposes
both addresses and requires the subscriber either to accept the
connection or to abort. Effectively, the subscriber device is
renumbered each time it connects for a new session.
For IPv6, the NCP is IP6CP [RFC5072] and is used to configure an
interface identifier for each end, after which link-local addresses
may be created in the normal way. In practice, each side can propose
its own identifier and renegotiation is only necessary when there is
a collision, or when a provider wishes to force a subscriber to use a
specific interface identifier. Once link-local addresses are
assigned and IP6CP is complete, automatic assignment of global scope
addresses is performed by the same methods as with multipoint links,
i.e., either SLAAC or DHCPv6. Again, in a subscriber/provider
environment, this allows renumbering per PPP session.
2.5. DNS Configuration
A site must provide DNS records for some or all of its hosts, and of
course these DNS records must be updated when hosts are renumbered.
Most sites will achieve this by maintaining a DNS zone file (or a
database from which it can be generated) and loading this file into
the site's DNS server(s) whenever it is updated. As a renumbering
tool, this is clumsy but effective. Clearly perfect synchronisation
between the renumbering of the host and the updating of its A or AAAA
record is impossible. An alternative is to use Secure Dynamic DNS
Update [RFC3007], in which a host informs its own DNS server when it
receives a new address.
There are widespread reports that the freely available BIND DNS
software (which is what most UNIX hosts use), Microsoft Windows (XP
and later), and Mac OS X all include support for Secure Dynamic DNS
Update. So do many home gateways. Further, there are credible
reports that these implementations are interoperable when configured
properly ([DNSBOOK] p. 228 and p. 506).
Commonly used commercial DNS and DHCP servers (e.g., Windows Server)
often are deployed with Secure Dynamic DNS Update also enabled. In
some cases, merely enabling both the DNS server and the DHCP server
might enable Secure Dynamic DNS Update as an automatic side effect
([DNSBOOK] p. 506). So in some cases, sites might have deployed
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Secure Dynamic DNS Update already, without realising it. An
additional enhancement would be for DHCP clients to implement support
for the "Client FQDN" option (Option 81).
Since address changes are usually communicated to other sites via the
DNS, the latter's security is essential for secure renumbering. The
Internet security community believes that the current DNS Security
(DNSsec) and Secure Dynamic DNS Update specifications are
sufficiently secure and has been encouraging DNSsec deployment
([RFC3007] [RFC4033] [RFC4034] [RFC4035]).
As of this writing, there appears to be significantly more momentum
towards rapid deployment of DNS Security standards in the global
public Internet than previously. Several country-code Top-Level
Domains (ccTLDs) have already deployed signed TLD root zones (e.g.,
Sweden's .SE). Several other TLDs are working to deploy signed TLD
root zones by published near-term deadlines (e.g., .GOV, .MIL). In
fact, it is reported that .GOV has been signed operationally since
early February 2009. It appears likely that the DNS-wide root zone
will be signed in the very near future. See, for example,
<http://www.dnssec-deployment.org/> and
<http://www.ntia.doc.gov/DNS/DNSSEC.html>.
2.6. Dynamic Service Discovery
The need for hosts to contain pre-configured addresses for servers
can be reduced by deploying the Service Location Protocol (SLP). For
some common services, such as network printing, SLP can therefore be
an important tool for facilitating site renumbering. See [RFC2608],
[RFC2610], [RFC3059], [RFC3224], [RFC3421], and [RFC3832].
Multicast DNS (mDNS) and DNS Service Discovery are already widely
deployed in BSD, Linux, Mac OS X, UNIX, and Windows systems, and are
also widely used for both link-local name resolution and for DNS-
based dynamic service discovery [MDNS] [DNSSD]. In many
environments, the combination of mDNS and DNS Service Discovery
(e.g., using SRV records [RFC3958]) can be important tools for
reducing dependency on configured addresses.
3. Existing Router-Related Mechanisms3.1. Router Renumbering
Although DHCP was originally conceived for host configuration, it can
also be used for some aspects of router configuration. The DHCPv6
Prefix Delegation options [RFC3633] are intended for this. For
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example, DHCPv6 can be used by an ISP to delegate or withdraw a
prefix for a customer's router, and this can be cascaded throughout a
site to achieve router renumbering.
An ICMPv6 extension to allow router renumbering for IPv6 is specified
in [RFC2894], but there appears to be little experience with it. It
is not mentioned as a useful mechanism by [RFC4192].
[RFC4191] extends IPv6 router advertisements to convey default router
preferences and more-specific routes from routers to hosts. This
could be used as an additional tool to convey information during
renumbering, but does not appear to be used in practice.
[CPE] requires that a customer premises router use DHCPv6 to obtain
an address prefix from its upstream ISP and use IPv6 RA messages to
establish a default IPv6 route (when IPv6 is in use).
4. Existing Multi-Addressing Mechanism for IPv6
IPv6 was designed to support multiple addresses per interface and
multiple prefixes per subnet. As described in [RFC4192], this allows
for a phased approach to renumbering (adding the new prefix and
addresses before removing the old ones).
As an additional result of the multi-addressing mechanism, a site
might choose to use Unique Local Addressing (ULA) [RFC4193] for all
on-site communication, or at least for all communication with on-site
servers, while using globally routeable IPv6 addresses for all off-
site communications. It would also be possible to use ULAs for all
on-site network management purposes, by assigning ULAs to all
devices. This would make these on-site activities immune to
renumbering of the prefix(es) used for off-site communication.
Finally, ULAs can be safely shared with peer sites with which there
is a VPN connection, which cannot be done with ambiguous IPv4
addresses [RFC1918]; such VPNs would not be affected by renumbering.
The IPv6 model also includes "privacy" addresses that are constructed
with pseudo-random interface identifiers to conceal actual MAC
addresses [RFC4941]. This means that IPv6 stacks and client
applications already need to be agile enough to handle frequent IP
address changes (e.g., in the privacy address), since in a privacy-
sensitive environment the address lifetime likely will be rather
short.
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RFC 5887 Renumbering Still Needs Work May 20105. Operational Issues with Renumbering Today
For IPv6, a useful description of practical aspects was drafted in
[THINK], as a complement to [RFC4192]. As indicated there, a primary
requirement is to minimise the disruption caused by renumbering.
This applies at two levels: disruption to site operations in general
and disruption to individual application sessions in progress at the
moment of renumbering. In the IPv6 case, the intrinsic ability to
overlap use of the old and new prefixes greatly mitigates disruption
to ongoing sessions, as explained in [RFC4192]. This approach is in
practice excluded for IPv4, largely because IPv4 lacks a Stateless
Address Autoconfiguration (SLAAC) mechanism.
5.1. Host-Related Issues5.1.1. Network-Layer Issues
For IPv4, the vast majority of client systems (PCs, workstations, and
handheld computers) today use DHCP to obtain their addresses and
other network-layer parameters. DHCP provides for lifetimes after
which the address lease expires. So it should be possible to devise
an operational procedure in which lease expiry coincides with the
moment of renumbering (within some margin of error). In the simplest
case, the network administrator just lowers all DHCP address lease
lifetimes to a very short value (e.g., a few minutes). It does this
long enough before a site-wide change that each node will
automatically pick up its new IP address within a few minutes of the
renumbering event. In this case, it would be the DHCP server itself
that automatically accomplishes client renumbering, although this
would cause a peak of DHCP traffic and therefore would not be
instantaneous. DHCPv6 could accomplish a similar result.
The FORCERENEW extension is defined for DHCP for IPv4 [RFC3203].
This is specifically unicast-only; a DHCP client must discard a
multicast FORCERENEW. This could nevertheless be used to trigger the
renumbering process, with the DHCP server cycling through all its
clients issuing a FORCERENEW to each one. DHCPv6 has a similar
feature, i.e., a unicast RECONFIGURE message, that can be sent to
each host to inform it to check its DHCPv6 server for an update.
These two features do not appear to be widely used for bulk
renumbering purposes.
Procedures for using a DHCP approach to site renumbering will be very
different depending on whether the site uses strong or weak asset
management. With strong asset management, and careful operational
planning, the subnet addresses and masks will be updated in the
database, and a script will be run to regenerate the DHCP MAC-to-IP
address tables and the DNS zone file. DHCP and DNS timers will be
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set temporarily to small values. The DHCP and DNS servers will be
fed the new files, and as soon as the previous DHCP leases and DNS
TTLs expire, everything will follow automatically, as far as the host
IP layer is concerned. In contrast, with weak asset management, and
a casual operational approach, the DHCP table will be reconfigured by
hand, the DNS zone file will be edited by hand, and when these
configurations are installed, there will be a period of confusion
until the old leases and TTLs expire. The DHCP FORCERENEW or
RECONFIGURE messages could shorten this confusion to some extent.
DHCP, particularly for IPv4, has acquired a very large number of
additional capabilities, with approximately 170 options defined at
the time of this writing. Although most of these do not carry IP
address information, some do (for example, options 68 through 76 all
carry various IP addresses). Thus, renumbering mechanisms involving
DHCP have to take into account more than the basic DHCP job of
leasing an address to each host.
SLAAC is much less overloaded with options than DHCP; in fact, its
only extraneous capability is the ability to convey a DNS server
address. Using SLAAC to force all hosts on a site to renumber is
therefore less complex than DHCP, and the difference between strong
and weak asset management is less marked. The principle of
synchronising the SLAAC and DNS updates, and of reducing the SLAAC
lease time and DNS TTL, does not change.
We should note a currently unresolved ambiguity in the interaction
between DHCPv6 and SLAAC from the host's point of view. RA messages
include a 'Managed Configuration' flag known as the M bit, which is
supposed to indicate that DHCPv6 is in use. However, it is
unspecified whether hosts must interpret this flag rigidly (i.e., may
or must only start DHCPv6 if it is set, or if no RAs are received) or
whether hosts are allowed or are recommended to start DHCPv6 by
default. An added complexity is that DHCPv6 has a 'stateless' mode
[RFC3736] in which SLAAC is used to obtain an address, but DHCPv6 is
used to obtain other parameters. Another flag in RA messages, the
'Other configuration' or O bit, indicates this.
Until this ambiguous behaviour is clearly resolved by the IETF,
operational problems are to be expected, since different host
operating systems have taken different approaches. This makes it
difficult for a site network manager to configure systems in such a
way that all hosts boot in a consistent way. Hosts will start SLAAC,
if so directed by appropriately configured RA messages. However, if
one operating system also starts a DHCPv6 client by default, and
another one starts it only when it receives the M bit, systematic
address management is impeded.
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Also, it should be noted that on an isolated LAN, neither RA nor
DHCPv6 responses will be received, and the host will remain with only
its self-assigned link-local address. One could also have a
situation where a multihomed network uses SLAAC for one address
prefix and DHCPv6 for another, which would clearly create a risk of
inconsistent host behaviour and operational confusion.
Neither the SLAAC approach nor DHCP without pre-registered MAC
addresses will work reliably in all cases of systems that are
assigned fixed IP addresses for practical reasons. Of course, even
systems with static addressing can be configured to use DHCP to
obtain their IP address(es). Such use of "Static DHCP" usually will
ease site renumbering when it does become necessary. However, in
other cases, manual or script-driven procedures, likely to be site-
specific and definitely prone to human error, are needed. If a site
has even one host with a fixed, manually configured address,
completely automatic host renumbering is very likely to be
impossible.
The above assumes the use of typical off-the-shelf hardware and
software. There are other environments, often referred to as
embedded systems, where DHCP or SLAAC might not be used and even
configuration scripts might not be an option; for example, fixed IP
addresses might be stored in read-only memory, or even set up using
Dual In-Line Package (DIP) switches. Such systems create special
problems that no general-purpose solution is likely to address.
5.1.2. Transport-Layer Issues
TCP connections and UDP flows are rigidly bound to a given pair of IP
addresses. These are included in the checksum calculation, and there
is no provision at present for the endpoint IP addresses to change.
It is therefore fundamentally impossible for the flows to survive a
renumbering event at either end. From an operational viewpoint, this
means that a site that plans to renumber itself is obliged either to
follow the overlapped procedure described in [RFC4192] or to announce
a site-wide outage for the renumbering process, during which all user
sessions will fail. In the case of IPv4, overlapping of the old and
new addresses is unlikely to be an option, and in any case is not
commonly supported by software. Therefore, absent enhancements to
TCP and UDP to enable dynamic endpoint address changes (for example,
[HANDLEY]), interruptions to TCP and UDP sessions seem inevitable if
renumbering occurs at either session endpoint. The same appears to
be true of Datagram Congestion Control Protocol (DCCP) [RFC4340].
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In contrast, Stream Control Transmission Protocol (SCTP) already
supports dynamic multihoming of session endpoints, so SCTP sessions
ought not be adversely impacted by renumbering the SCTP session
endpoints [RFC4960] [RFC5061].
5.1.3. DNS Issues
The main issue is whether the site in question has a systematic
procedure for updating its DNS configuration. If it does, updating
the DNS for a renumbering event is essentially a clerical issue that
must be coordinated as part of a complete plan, including both
forward and reverse mapping. As mentioned in [RFC4192], the DNS TTL
will be manipulated to ensure that stale addresses are not cached.
However, if the site uses a weak asset management model in which DNS
updates are made manually on demand, there will be a substantial
period of confusion and errors will be made.
There are anecdotal reports that many small user sites do not even
maintain their own DNS configuration, despite running their own web
and email servers. They point to their ISP's resolver, request the
ISP to install DNS entries for their servers, but operate internally
mainly by IP address. Thus, renumbering for such sites will require
administrative coordination between the site and its ISP(s), unless
the DNS servers in use have Secure Dynamic DNS Update enabled. Some
commercial DNS service firms include Secure Dynamic DNS Update as
part of their DNS service offering.
It should be noted that DNS entries commonly have matching Reverse
DNS entries. When a site renumbers, these reverse entries will also
need to be updated. Depending on a site's operational arrangements
for DNS support, it might or might not be possible to combine forward
and reverse DNS updates in a single procedure.
5.1.4. Application-Layer Issues
Ideally, we would carry out a renumbering analysis for each
application protocol. To some extent, this has been done, in
[RFC3795]. This found that 34 out of 257 Standards-Track or
Experimental application-layer RFCs had explicit address
dependencies. Although this study was made in the context of IPv4 to
IPv6 transition, it is clear that all these protocols might be
sensitive to renumbering. However, the situation is worse, in that
there is no way to discover by analyzing specifications whether an
actual implementation is sensitive to renumbering. Indeed, such
analysis might be quite impossible in the case of proprietary
applications.
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The sensitivity depends on whether the implementation stores IP
addresses in such a way that it might refer back to them later,
without allowing for the fact that they might no longer be valid. In
general, we can assert that any implementation is at risk from
renumbering if it does not check that an address is valid each time
it opens a new communications session. This could be done, for
example, by knowing and respecting the relevant DNS TTL, or by
resolving relevant Fully-Qualified Domain Names (FQDNs) again. A
common experience is that even when FQDNs are stored in configuration
files, they are resolved only once, when the application starts, and
they are cached indefinitely thereafter. This is insufficient. Of
course, this does not apply to all application software; for example,
several well-known web browsers have short default cache lifetimes.
There are even more egregious breaches of this principle, for
example, software license systems that depend on the licensed host
computer having a particular IP address. Other examples are the use
of literal IP addresses in URLs, HTTP cookies, or application proxy
configurations. (Also see Appendix A.)
In contrast, there are also many application suites that actively
deal with connectivity failures by retrying with alternative
addresses or by repeating DNS lookups. This places a considerable
burden of complexity on application developers.
It should be noted that applications are in effect encouraged to be
aware of and to store IP addresses by the very nature of the socket
API calls gethostbyname() and getaddrinfo(). It is highly
unfortunate that many applications use APIs that require the
application to see and use lower-layer objects, such as network-layer
addresses. However, the BSD Sockets API was designed and deployed
before the Domain Name System (DNS) was created, so there were few
good options at the time. This issue is made worse by the fact that
these functions do not return an address lifetime, so that
applications have no way to know when an address is no longer valid.
The extension of the same model to cover IPv6 has complicated this
problem somewhat. An application using the socket API is obliged to
contain explicit logic if it wishes to benefit from the availability
of multiple addresses for a given remote host. If a programming
model were adopted in which only FQDNs were exposed to applications,
and addresses were cached with appropriate lifetimes within the API,
most of these problems would disappear. It should be noted that at
least the first part of this is already available for some
programming environments. A common example is Java, where only FQDNs
need to be handled by application code in normal circumstances, and
no additional application logic is needed to deal with multiple
addresses, which are handled by the run-time system. This is highly
beneficial for programmers who are not networking experts, and
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insulates applications software from many aspects of renumbering. It
would be helpful to have similarly abstract, DNS-oriented, Networking
APIs openly specified and widely available for C and C++.
Some web browsers intentionally violate the DNS TTL with a technique
called "DNS Pinning." DNS Pinning limits acceptance of server IP
address changes, due to a JavaScript issue where repeated address
changes can be used to induce a browser to scan the inside of a
firewalled network and report the results to an outside attacker.
Pinning can persist as long as the browser is running, in extreme
cases perhaps months at a time. Thus, we can see that security
considerations may directly damage the ability of applications to
deal with renumbering.
Server applications might need to be restarted when the host they
contain is renumbered, to ensure that they are listening on a port
and socket bound to the new address. In an IPv6 multi-addressed
host, server applications need to be able to listen on more than one
address simultaneously, in order to cover an overlap during
renumbering. Not all server applications are written to do this, and
a name-based API as just mentioned would have to provide for this
case invisibly to the server code.
As noted in Section 2.6, the Service Location Protocol (SLP), and
multicast DNS with SRV records for service discovery, have been
available for some years. For example, many printers deployed in
recent years automatically advertise themselves to potential clients
via SLP. Many modern client operating systems automatically
participate in SLP without requiring users to enable it. These tools
appear not to be widely known, although they can be used to reduce
the number of places that IP addresses need to be configured.
5.2. Router-Related Issues
[RFC2072] gives a detailed review of the operational realities in
1997. A number of the issues discussed in that document were the
result of the relatively recent adoption of classless addressing;
those issues can be assumed to have vanished by now. Also, DHCP was
a relative newcomer at that time, and can now be assumed to be
generally available. Above all, the document underlines that
systematic planning and administrative preparation are needed, and
that all forms of configuration file and script must be reviewed and
updated. Clearly this includes filtering and routing rules (e.g.,
when peering with BGP, but also with intradomain routing as well).
Two particular issues mentioned in [RFC2072] are:
o Some routers cache IP addresses in some situations. So routers
might need to be restarted as a result of site renumbering.
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o Addresses might be used by configured tunnels, including VPN
tunnels, although at least the Internet Key Exchange (IKE)
supports the use of Fully-Qualified Domain Names instead.
On the latter point, the capability to use FQDNs as endpoint names in
IPsec VPNs is not new and is standard (see [RFC2407], Section 4.6.2.3
and [RFC4306], Section 3.5). This capability is present in most
IPsec VPN implementations. There does seem to be an "educational" or
"awareness" issue that many system/network administrators do not
realise that it is there and works well as a way to avoid manual
modification during renumbering. (Of course, even in this case, a
VPN may need to be reconnected after a renumbering event, but most
products appear to handle this automatically.)
In IPv6, if a site wanted to be multihomed using multiple provider-
aggregated (PA) routing prefixes with one prefix per upstream
provider, then the interior routers would need a mechanism to learn
which upstream providers and prefixes were currently reachable (and
valid). In this case, their Router Advertisement messages could be
updated dynamically to only advertise currently valid routing
prefixes to hosts. This would be significantly more complicated if
the various provider prefixes were of different lengths or if the
site had non-uniform subnet prefix lengths.
5.3. Other Issues5.3.1. NAT State Issues
When a renumbering event takes place, entries in the state table of
any Network Address Translator that happen to contain the affected
addresses will become invalid and will eventually time out. Since
TCP and UDP sessions are unlikely to survive renumbering anyway, the
hosts involved will not be additionally affected. The situation is
more complex for multihomed SCTP [SCTPNAT], depending on whether a
single or multiple NATs are involved.
A NAT itself might be renumbered and might need a configuration
change during a renumbering event. One of the authors has a NAT-
enabled home gateway that obtains its exterior address from the
residential Internet service provider by acting as a DHCP client.
That deployment has not suffered operational problems when the ISP
uses DHCP to renumber the gateway's exterior IP address. A critical
part of that success has been configuring IKE on the home gateway to
use a "mailbox name" for the user's identity type (rather than using
the exterior IP address of the home gateway) when creating or
managing the IP Security Associations.
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RFC 5887 Renumbering Still Needs Work May 20105.3.2. Mobility Issues
A mobile node using Mobile IP that is not currently in its home
network will be adversely affected if either its current care-of
address or its home address is renumbered. For IPv6, if the care-of
address changes, this will be exactly like moving from one foreign
network to another, and Mobile IP will re-bind with its home agent in
the normal way. If its home address changes unexpectedly, it can be
informed of the new global routing prefix used at the home site
through the Mobile Prefix Solicitation and Mobile Prefix
Advertisement ICMPv6 messages [RFC3775]. The situation is more
tricky if the mobile node is detached at the time of the renumbering
event, since it will no longer know a valid subnet anycast address
for its home agent, leaving it to deduce a valid address on the basis
of DNS information.
In contrast to Mobile IPv6, Mobile IPv4 does not support prefix
solicitation and prefix advertisement messages, limiting its
renumbering capability to well-scheduled renumbering events when the
mobile node is connected to its home agent and managed by the home
network administration. Unexpected home network renumbering events
when the mobile node is away from its home network and not connected
to the home agent are supported only if a relevant Authentication,
Authorisation, and Accounting (AAA) system is able to allocate
dynamically a home address and home agent for the mobile node.
5.3.3. Multicast Issues
As discussed in [THINK], IPv6 multicast can be used to help rather
than hinder renumbering, for example, by using multicast as a
discovery protocol (as in IPv6 Neighbour Discovery). On the other
hand, the embedding of IPv6 unicast addresses into multicast
addresses specified in [RFC3306] and the embedded-RP (Rendezvous
Point) in [RFC3956] will cause issues when renumbering.
For both IPv4 and IPv6, changing the unicast source address of a
multicast sender might also be an issue for receivers, especially for
Source-Specific Multicast (SSM). Applications need to learn the new
source addresses and new multicast trees need to be built
For IPv4 or IPv6 with Any-Source Multicast (ASM), renumbering can be
easy. If sources are renumbered, from the routing perspective,
things behave just as if there are new sources within the same
multicast group. There may be application issues though. Changing
the RP address is easy when using Bootstrap Router (BSR) [RFC5059]
for dynamic RP discovery. BSR is widely used, but it is also common
to use static config of RP addresses on routers. In that case,
router configurations must be updated too.
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If any multicast ACLs are configured, they raise the same issue as
unicast ACLs mentioned elsewhere.
5.3.4. Management Issues
Today, static IP addresses are routinely embedded in numerous
configuration files and network management databases, including MIB
modules. Ideally, all of these would be generated from a single
central asset management database for a given site, but this is far
from being universal practice. It should be noted that for IPv6,
where multiple routing prefixes per interface and multiple addresses
per interface are standard practice, the database and the
configuration files will need to allow for this (rather than for a
single address per host, as is normal practice for IPv4).
Furthermore, because of routing policies and VPNs, a site or network
might well embed addresses from other sites or networks in its own
configuration data. (It is preferable to embed FQDNs instead, of
course, whenever possible.) Thus, renumbering will cause a ripple
effect of updates for a site and for its neighbours. To the extent
that these updates are manual, they will be costly and prone to
error. Synchronising updates between independent sites can cause
unpredictable delays. Note that Section 4 suggests that IPv6 ULAs
can mitigate this problem, but of course only for VPNs and routes
that are suitable for ULAs rather than globally routeable addresses.
The majority of external addresses to be configured will not be ULAs.
See Appendix A for an extended list of possible static or embedded
addresses.
Some address configuration data are relatively easy to find (for
example, site firewall rules, ACLs in site border routers, and DNS).
Others might be widely dispersed and much harder to find (for
example, configurations for building routers, printer addresses
configured by individual users, and personal firewall
configurations). Some of these will inevitably be found only after
the renumbering event, when the users concerned encounter a problem.
The overlapped model for IPv6 renumbering, with old and new addresses
valid simultaneously, means that planned database and configuration
file updates will proceed in two stages -- add the new information
some time before the renumbering event, and remove the old
information some time after. All policy rules must be configured to
behave correctly during this process (e.g., preferring the new
address as soon as possible). Similarly, monitoring tools must be
set up to monitor both old and new during the overlap.
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However, it should be noted that the notion of simultaneously
operating multiple prefixes for the same network, although natural
for IPv6, is generally not supported by operational tools such as
address management software. It also increases the size of IGP
routing tables in proportion to the number of prefixes in use. For
these reasons, and due to its unfamiliarity to operational staff, the
use of multiple prefixes remains rare. Accordingly, the use of ULAs
to provide stable on-site addresses even if the off-site prefix
changes is also rare.
If both IPv4 and IPv6 are renumbered simultaneously in a dual-stack
network, additional complications could result, especially with
configured IP-in-IP tunnels. This scenario should probably be
avoided.
Use of FQDNs rather than raw IP addresses wherever possible in
configuration files and databases will reduce/mitigate the potential
issues arising from such configuration files or management databases
when renumbering is required or otherwise occurs. This is advocated
in [RFC1958] (point 4.1). Just as we noted in Section 5.1.4 for
applications, this is insufficient in itself; some devices such as
routers are known to only resolve FQDNs once, at start-up, and to
continue using the resulting addresses indefinitely. This may
require routers to be rebooted, when they should instead be resolving
the FQDN again after a given timeout.
By definition, there is at least one place (i.e., the DNS zone file
or the database from which it is derived) where address information
is nevertheless inevitable.
It is also possible that some operators may choose to configure
addresses rather than names, precisely to avoid a possible circular
dependency and the resulting failure modes. This is arguably even
advocated in [RFC1958] (point 3.11).
It should be noted that the management and administration issues
(i.e., tracking down, recording, and updating all instances where
addresses are stored rather than looked up dynamically) form the
dominant concern of managers considering the renumbering problem.
This has led many sites to continue the pre-CIDR (Classless Inter-
Domain Routing) approach of using a provider-independent (PI) prefix.
Some sites, including very large corporate networks, combine PI
addressing with NAT. Others have adopted private addressing and NAT
as a matter of choice rather than obligation. This range of
techniques allows for addressing plans that are independent of the
ISP(s) in use, and allows a straightforward approach to multihoming.
The direct cost of renumbering is perceived to exceed the indirect
costs of these alternatives. Additionally, there is a risk element
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stemming from the complex dependencies of renumbering: it is hard to
be fully certain that the renumbering will not cause unforeseen
service disruptions, leading to unknown additional costs.
A relevant example in a corporate context is VPN configuration data
held in every employee laptop, for use while on travel and connecting
securely from remote locations. Typically, such VPNs are statically
configured using numeric IP addresses for endpoints and even with
prefix lists for host routing tables. Use of VPN configurations with
FQDNs to name fixed endpoints, such as corporate VPN gateways, and
with non-address identity types would enable existing IP Security
with IKE to avoid address renumbering issues and work well for highly
mobile users. This is all possible today with standard IPsec and
standard IKE. It just requires VPN software to be configured with
names instead of addresses, and thoughtful network administration.
It should be noted that site and network operations managers are
often very conservative, and reluctant to change operational
procedures that are working reasonably well and are perceived as
reasonably secure. They quite logically argue that any change brings
with it an intrinsic risk of perturbation and insecurity. Thus, even
if procedural changes are recommended that will ultimately reduce the
risks and difficulties of renumbering (such as using FQDNs protected
by DNSsec where addresses are used today), these changes might be
resisted.
5.3.5. Security Issues
For IPv6, addresses are intended to be protected against forgery
during neighbour discovery by SEcure Neighbour Discovery (SEND)
[RFC3971]. This appears to be a very useful precaution during
dynamic renumbering, to prevent hijacking of the process by an
attacker. Any automatic renumbering scheme has a potential exposure
to such hijacking at the moment that a new address is announced.
However, at present it is unclear whether or when SEND might be
widely implemented or widely deployed.
Firewall rules will certainly need to be updated, and any other cases
where addresses or address prefixes are embedded in security
components (access control lists, AAA systems, intrusion detection
systems, etc.). If this is not done in advance, legitimate access to
resources might be blocked after the renumbering event. If the old
rules are not removed promptly, illegitimate access might be possible
after the renumbering event. Thus, the security updates will need to
be made in two stages (immediately before and immediately after the
event).
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There will be operational and security issues if an X.509v3 Public
Key Infrastructure (PKI) Certificate includes a subjectAltName
extension that contains an iPAddress [RFC5280], and if the
corresponding node then undergoes an IP address change without a
concurrent update to the node's PKI Certificate. For these reasons,
use of the dNSName rather than the iPAddress is recommended for the
subjectAltName extension. Any other use of IP addresses in
cryptographic material is likely to be similarly troublesome.
If a site is, for some reason, listed by IP address in a white list
(such as a spam white list), this will need to be updated.
Conversely, a site that is listed in a black list can escape that
list by renumbering itself.
The use of IP addresses instead of FQDNs in configurations is
sometimes driven by a perceived security need. Since the name
resolution process has historically lacked authentication,
administrators prefer to use raw IP addresses when the application is
security sensitive (firewalls and VPN are two typical examples). It
might be possible to solve this issue in the next few years with
DNSsec (see Section 2.5), now that there is strong DNS Security
deployment momentum.
6. Proposed Mechanisms6.1. SHIM6
SHIM6, proposed as a host-based multihoming mechanism for IPv6, has
the property of dynamically switching the addresses used for
forwarding the actual packet stream while presenting a constant
address as the upper-layer identifier for the transport layer
[RFC5533]. At least in principle, this property could be used during
renumbering to alleviate the problem described in Section 5.1.2.
SHIM6 is an example of a class of solutions with this or a similar
property; others are Host Identity Protocol (HIP), IKEv2 Mobility and
Multihoming (MOBIKE), Mobile IPv6, SCTP, and proposals for multi-path
TCP.
6.2. MANET Proposals
The IETF working groups dealing with mobile ad hoc networks have been
working on a number of mechanisms for mobile routers to discover
available border routers dynamically, and for those mobile routers to
be able to communicate that information to hosts connected to those
mobile routers.
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Recently, some MANET work has appeared on a "Border Router Discovery
Protocol (BRDP)" that might be useful work towards a more dynamic
mechanism for site interior router renumbering [BRDP].
At present, the IETF AUTOCONF WG
(http://www.ietf.org/html.charters/autoconf-charter.html) is working
on address autoconfiguration mechanisms for MANET networks that also
seem useful for ordinary non-mobile non-MANET networks [AUTOC]. This
work is extensively surveyed in [AUTOC2] and [AUTOC3]. Other work in
the same area, e.g., [RFC5558], might also be relevant.
MANETs are, of course, unusual in that they must be able to
reconfigure themselves at all times and without notice. Hence, the
type of hidden static configurations discussed above in Section 5.3.4
are simply intolerable in MANETs. Thus, it is possible that when a
consensus is reached on autoconfiguration for MANETs, the selected
solution(s) might not be suitable for the more general renumbering
problem. However, it is certainly worthwhile to explore applying
techniques that work for MANETs to conventional networks also.
6.3. Other IETF Work
A DHCPv6 extension has been proposed that could convey alternative
routes, in addition to the default router address, to IPv6 hosts
[DHRTOPT]. Other DHCP options are also on the table that may offer
information about address prefixes and routing to DHCP or DHCPv6
clients, such as [DHSUBNET] and [DHMIFRT]. It is conceivable that
these might be extended as a way of informing hosts dynamically of
prefix changes.
In the area of management tools, Network Configuration (NETCONF)
Protocol [RFC4741] is suitable for the configuration of any network
element or server, so could in principle be used to support secure
remote address renumbering.
The DNSOP WG has considered a Name Server Control Protocol (NSCP)
based on NETCONF that provides means for consistent DNS management
including potential host renumbering events [DNSCONT].
6.4. Other Proposals
A proposal has been made to include an address lifetime as an
embedded field in IPv6 addresses, with the idea that all prefixes
would automatically expire after a certain period and become
unrouteable [CROCKER]. While this might be viewed as provocative, it
would force the issue by making renumbering compulsory.
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RFC 5887 Renumbering Still Needs Work May 20107. Gaps
This section seeks to identify technology gaps between what is
available from existing open specifications and what appears to be
needed for site renumbering to be tolerable.
7.1. Host-Related Gaps
It would be beneficial to expose address lifetimes in the socket API,
or any low-level networking API. This would allow applications to
avoid using stale addresses.
The various current discussions of a name-based transport layer or a
name-based network API also have potential to alleviate the
application-layer issues noted in this document. Application
development would be enhanced by the addition of a more abstract
network API that supports the C and C++ programming languages. For
example, it could use FQDNs and Service Names, rather than SockAddr,
IP Address, protocol, and port number. This would be equivalent to
similar interfaces already extant for Java programmers.
Moving to a FQDN-based transport layer might enhance the ability to
migrate the IP addresses of endpoints for TCP/UDP without having to
interrupt a session, or at least in a way that allows a session to
restart gracefully.
Having a single registry per host for all address-based configuration
(/etc/hosts, anyone?), with secure access for site network
management, might be helpful. Ideally, this would be remotely
configurable, for example, leveraging the IETF's current work on
networked-device configuration protocols (NetConf). While there are
proprietary versions of this approach, sometimes based on Lightweight
Directory Access Protocol (LDAP), a fully standardised approach seems
desirable.
Do we really need more than DHCP or SLAAC for regular hosts? Do we
need an IPv4 equivalent of SLAAC? How can the use of DHCP FORCERENEW
and DHCPv6 RECONFIGURE for bulk renumbering be deployed? FORCERENEW
in particular requires DHCP authentication [RFC3118] to be deployed.
The IETF should resolve the 'IPv6 ND M/O flag debate' once and for
all, with default, mandatory and optional behaviours of hosts being
fully specified.
The host behaviour for upstream link learning suggested in
Section 2.3 should be documented.
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It would be helpful to have multi-path, survivable, extensions for
both UDP and TCP (or institutionalise some aspects of SHIM6).
7.2. Router-Related Gaps
A non-proprietary secure mechanism to allow all address-based
configuration to be driven by a central repository for site
configuration data. NETCONF might be a good starting point.
A MANET solution that's solid enough to apply to fully operational
small to medium fixed sites for voluntary or involuntary renumbering.
A MANET-style solution that can be applied convincingly to large or
very large sites for voluntary renumbering.
A useful short-term measure would be to ensure that [RFC2894] and
[RFC3633] can be used in practice.
7.3. Operational Gaps
Since address changes are usually communicated via the DNS, the
latter's security is essential for secure renumbering. Thus, we
should continue existing efforts to deploy DNSsec globally, including
not only signing the DNS root, DNS TLDs, and subsidiary DNS zones,
but also widely deploying the already available DNSsec-capable DNS
resolvers.
Similarly, we should document and encourage widespread deployment of
Secure Dynamic DNS Update both in DNS servers and also in both client
and server operating systems. This capability is already widely
implemented and widely available, but it is not widely deployed at
present.
Deploy multi-prefix usage of IPv6, including Unique Local Addresses
(ULAs) to provide stable internal addresses. In particular, address
management tools need to support the multi-prefix model and ULAs.
Ensure that network monitoring systems will function during
renumbering, in particular to confirm that renumbering has completed
successfully or that some traffic is still using the old prefixes.
Document and encourage systematic site databases and secure
configuration protocols for network elements and servers (e.g.,
NETCONF). The database should store all the information about the
network; scripts and tools should derive all configurations from the
database. An example of this approach to simplify renumbering is
given at [LEROY].
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Document functional requirements for site renumbering tools or
toolkits.
Document operational procedures useful for site renumbering.
In general, document renumbering instructions as part of every
product manual.
Recommend strongly that all IPv6 deployment plans, for all sizes of
site or network, should include provision for future renumbering.
Renumbering should be planned from day one when the first lines of
the configuration of a network or network service are written. Every
IPv6 operator should expect to have to renumber the network one day
and should plan for this event.
7.4. Other Gaps
Define a secure mechanism for announcing changes of site prefix to
other sites (for example, those that configure routers or VPNs to
point to the site in question).
For Mobile IP, define a better mechanism to handle change of home
agent address while mobile is disconnected.
8. Security Considerations
Known current issues are discussed in Section 5.3.5. Security issues
related to SLAAC are discussed in [RFC3756]. DHCP authentication is
defined in [RFC3118].
For future mechanisms to assist and simplify renumbering, care must
be taken to ensure that prefix or address changes (especially changes
coming from another site or via public sources such as the DNS) are
adequately authenticated at all points. Otherwise, misuse of
renumbering mechanisms would become an attractive target for those
wishing to divert traffic or to cause major disruption. As noted in
Section 5.1.4, this may result in defensive techniques such as "DNS
pinning", which create difficulty when renumbering.
Whatever authentication method(s) are adopted, key distribution will
be an important aspect. Most likely, public key cryptography will be
needed to authenticate renumbering announcements passing from one
site to another, since one cannot assume a preexisting trust
relationship between such sites.
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[SIX-ONE] Vogt, C., "Six/One: A Solution for Routing and
Addressing in IPv6", Work in Progress, October 2009.
[THINK] Chown, T., "Things to think about when Renumbering an
IPv6 network", Work in Progress, September 2006.
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